The present invention relates to a new method for forming graph scales on the panel of a CRT for oscilloscope, in method for forming graph scales on the panel by Exposition & Development System being characteristic of placing between a negative mask for the graph scale patterns and a light source a specific filter whose light transmissivity at its center is about 62% and gets larger along from its center to its peripheral portions.
In general, for example as shown in FIG. 1, the graph scale patterns including the scales for graph coordinates 1 and other necessary numerical values 2 are drawn on the surface of the panel(P) of the CRT for a oscilloscope. In the past, these graph scale patterns have been made by the method of attaching on the panel a transparent plastic board on which the necessary graph patterns are provided, but recently either by one method in which the graph patterns are directly drawn on the panel surface before complishing a glass bulb consisting of a Panel part and a Funnel part or by another method of forming the necessary graph scale patterns on the inner surface of the panel by light exposure and development means well known to the CRT manufacturers. In the above, the latter is preferred above all owing to its lower cost and the advantage in recycling the glass bulb.
One recent method frequently used by CRT manufacturers is as follows:
The inside of a glass bulb, which is arranged so that its panel part is downward, is filled with a slurry comprising suitable pigment, water, photo-sensitive agents and so on. Negative mask(M), a film on which a negative graph scale patterns 1' are provided for example as shown in FIG. 2, is arranged outside the panel part in parallel to the panel part. Then, the necessary graph scale patterns are obtained by exposing and developing the slurry precipitates deposited on the inner surface of the panel part of the bulb by the light from a light source.
The above-mentioned method has the problems as follows;
1. The mercury lamp generally used in this method does not provide monochromatic light beams but provides light beams of various wavelengths. Therefore, it is difficult to obtain correct graph scale patterns since the refractive index of the light beams passing through the collimating lens means are different from each other.
2. The light is likely to disperse when it passes through the lens means since the lamp is not a point-light source but is a surface-light source.
3. The light also disperses due to the ununiformity of the quality of the lens means themselves and the intensity of light drops due to the light transmissivity of each of the lens means.
4. The distance from the light source to the panel gets too far since a large number of lens means should be used in order to get correct collimating light beams. As a result, the intensity of the light is significantly reduced when it reaches the slurry precipitates.
In the above, if the graph scale patterns made without the lens means so as to solve the above-mentioned problems involved in using them, the obtained graph scale patterns may be different from the intended ones because of the odds of the incident angles of light arrived at each position of the mask and the panel. Also the ideal pattern lines with a uniform thickness can not be obtained due to the odds of the intensity of light arriving at each parts of the panel.